Transformer and dcdc converter

Abstract

The present invention increases leakage inductance of a transformer. A transformer (1) includes: a core (10) having a first base part (11), a second base part (12), and leg parts (21) to (24); a primary winding group (31) and a secondary winding group (32) wound around the core (10). The leg parts (21) to (24) are disposed at intervals in a second direction (x2) that intersects a first direction (x1) in which the first base part (11) and the second base part (12) are opposed to each other. The primary winding group (31) includes primary coils (41) to (44), and the secondary winding group (32) includes secondary coils (51) to (54). The primary coils (41) to (44) are wound correspondingly to the leg parts (21) to (24) and are connected in series. Winding directions of the two primary coils corresponding to the two leg parts adjacent to each other in the second direction (x2) are opposite to each other. The primary coil and the secondary coil are wound around each of the two leg parts of the leg parts (21) to (24), and the other primary coils are wound around each of the other two leg parts.

Description

Transformers and DC-DC converters

[0001] The present disclosure relates to a transformer having a primary winding and a secondary winding, and a DC-DC converter including the transformer.

[0002] Conventionally, transformers having a primary winding and a secondary winding are known. In the transformer disclosed in Patent Document 1, leakage inductance is obtained by generating magnetic fluxes with different magnitudes at predetermined locations on the core.

[0003] Japanese Patent Application Laid-Open No. 2017-73418

[0004] However, in the transformer described in Patent Document 1, magnetic fluxes with different magnitudes can be generated only at predetermined locations in the core, making it difficult to obtain a large leakage inductance.

[0005] The present disclosure provides a transformer and the like that can increase leakage inductance.

[0006] A transformer according to one aspect of the present disclosure includes a first base, a second base provided opposite the first base, a core having four or more legs provided between the first base and the second base, and a primary winding group and a secondary winding group wound around the core, wherein the four or more legs are arranged at intervals in a second direction intersecting a first direction in which the first base and the second base face each other, the primary winding group includes four or more primary coils, the secondary winding group includes two or more secondary coils, and the four The above primary coils are wound corresponding to the four or more legs and connected in series, and the winding directions of the two primary coils wound corresponding to two adjacent legs in the second direction are opposite to each other, and the primary coil and the secondary coil are wound around each of two or more of the four or more legs, and another primary coil different from the primary coil is wound around each of two or more other legs different from the two or more legs among the four or more legs.

[0007] A DC-DC converter according to one aspect of the present disclosure includes the above-described transformer and a resonant capacitor connected in series to the transformer.

[0008] A DC-DC converter according to one aspect of the present disclosure includes the above-described transformer, an external inductor connected in series to the transformer, and a resonant capacitor connected in series to the external inductor.

[0009] According to the transformer etc. of the present disclosure, leakage inductance can be increased.

[0010] 1 is a circuit showing an LLC converter of a comparative example; FIG. 2 is a diagram showing an example of gain characteristics of the LLC converter of the comparative example; FIG. 3 is a diagram showing an example of a combination of cores constituting a transformer; FIG. 4 is a circuit showing an LLC converter of an embodiment; FIG. 5 is an external view of a transformer according to an embodiment; FIG. 6 is a diagram showing a core of a transformer according to an embodiment; FIG. 7 is a schematic diagram of a transformer according to an embodiment; FIG. 8 is a diagram showing magnetic flux generated in a transformer; FIG. 9 is a diagram showing an example of gain characteristics of an LLC converter using a transformer; FIG. 10 is a schematic diagram of a transformer according to a first modification of an embodiment; FIG. 11 is a schematic diagram of a transformer according to a second modification of an embodiment; FIG. 12 is a schematic diagram of a transformer according to a third modification of an embodiment; FIG. 13 is a schematic diagram showing an example of a transformer according to a fourth modification of an embodiment; FIG. 14 is a schematic diagram showing another example of a transformer according to the fourth modification of an embodiment; FIG. 15 is a diagram showing a core of a transformer according to a fifth modification of an embodiment; FIG. 16 is a schematic diagram showing an example of a transformer according to a sixth modification of an embodiment; FIG. 17 is a schematic diagram showing another example of a transformer according to the sixth modification of an embodiment; FIG. 10 is a schematic diagram showing another example of a transformer according to a seventh modification of the embodiment. FIG. 11 is a schematic diagram showing a transformer according to a ninth modification of the embodiment. FIG. 12 is a schematic diagram showing a magnetic flux generated around a winding when a current flows. FIG. 13 is a schematic diagram showing a relationship between the length of a leg of a core and copper loss. FIG. 14 is a schematic diagram showing an example of a transformer according to a tenth modification of the embodiment. FIG. 15 is a schematic diagram showing another example of a transformer according to the tenth modification of the embodiment. FIG. 16 is a schematic diagram showing another example of a transformer according to an eleventh modification of the embodiment. FIG. 17 is a schematic diagram showing an example of a transformer according to a twelfth modification of the embodiment. FIG. 18 is a schematic diagram showing another example of a transformer according to the thirteenth modification of the embodiment. FIG. 19 is a schematic diagram showing a conductor pattern and via conductors of a transformer according to the thirteenth modification of the embodiment. FIG. 19 is a schematic diagram showing a transformer according to a fourteenth modification of the embodiment. FIG. 19 is a schematic diagram showing a conductor pattern and via conductors of a transformer according to the fourteenth modification of the embodiment. 1 is a circuit diagram showing another example of an LLC converter according to an embodiment of the present invention;

[0011] (Background to the Invention of the Disclosure) The background to the invention of the disclosure will be described with reference to FIGS. 1 to 3. FIG.

[0012] FIG. 1 is a circuit diagram showing an example of an LLC converter 105 of a comparative example.

[0013] 1 shows a transformer, a switch circuit, etc. The figure also shows an example of an input voltage Vin, an output voltage Vout, and a turns ratio N. The circuit also shows the magnetizing inductance Lm, leakage inductance Lr, and capacitance Cr of the resonant capacitor of the transformer, which are shown as an equivalent circuit.

[0014] FIG. 2 is a diagram showing an example of the gain characteristic of the LLC converter 105 of the comparative example.

[0015] The horizontal axis of the figure represents normalized frequency, and the vertical axis represents a transfer function that is the gain of the LLC converter 105. In the figure, m represents the ratio between the excitation inductance Lm and the leakage inductance Lr, and is calculated by m=Lm / Lr (Equation 1).

[0016] Power supplies for automotive accessories are required to operate over a wide range of input and output voltages. For this reason, for example, a transformer with a large turns ratio N is used to increase the step-down ratio, and the gain (input / output voltage ratio) is adjusted by changing the operating frequency.

[0017] As shown in Figure 2, the smaller the ratio m, the wider the range of gain change relative to frequency change. Based on the above-mentioned (Equation 1), the ratio m can be reduced by increasing the leakage inductance Lr, for example. Therefore, in order to achieve wide-range operation in LLC converter 105, it is considered necessary to increase the leakage inductance Lr of the transformer to a certain extent.

[0018] For example, a large leakage inductance Lr can be obtained by using a plurality of transformers, but the use of a plurality of transformers results in a problem in that the overall area of ​​the transformers becomes large.

[0019] 3 is a diagram showing an example of a combination of cores that make up a transformer, in which the primary winding is indicated by a solid arrow and the secondary winding is indicated by a dashed arrow.

[0020] Figure 3(a) shows a transformer consisting of two core sets: a concave-convex E-core with three legs and a flat I-core. As shown in the figure, simply using two core sets would increase the overall area of ​​the transformer.

[0021] Figure 3(b) shows a transformer in which adjacent legs of two sets of cores are joined together to form an integrated unit. In this example, the joined legs are used as the center legs, and the primary winding is wound so that the magnetic flux is canceled at the center legs. As shown in the figure, the area of ​​the transformer is reduced by the amount of integration, but the overall area is still large.

[0022] Figure 3(c) shows an example in which the center leg has been removed from the transformer shown in Figure 3(b). Because the center leg is the leg where magnetic flux is canceled, removing it does not substantially reduce functionality. By removing the center leg as shown in Figure 3(c), it is believed that a compact transformer with the same functionality as the transformer shown in Figure 3(a) can be realized.

[0023] Based on these ideas, the present disclosure proposes a transformer or the like that is small in size yet can achieve large leakage inductance.

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that each of the embodiments described below represents a specific example of the present disclosure. Therefore, the numerical values, shapes, materials, components, component placement positions, connection forms, etc. shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Therefore, among the components in the following embodiments, components not recited in independent claims will be described as optional components.

[0025] Note that each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, the scales and the like do not necessarily match in each figure. Furthermore, in each figure, the same reference numerals are used for substantially the same configurations, and redundant explanations will be omitted or simplified.

[0026] Each figure also shows x1, x2, and x3 axes, which represent three mutually orthogonal directions, and these axes and the axial directions along these axes are used as necessary for explanation. Note that these axes are added for explanation purposes only and do not limit the direction or posture in which the transformer is used.

[0027] (Embodiment) [Configuration of LLC Converter] An LLC converter including a transformer according to an embodiment will be described. The LLC converter is an example of a DC-DC converter.

[0028] FIG. 4 is a circuit diagram showing the LLC converter 5 according to the embodiment.

[0029] The LLC converter 5 includes a primary circuit 6, a transformer 1, and a secondary circuit 7. The LLC converter 5 also includes a resonant capacitor C11. The resonant capacitor C11 is provided on a path connecting the primary circuit 6 and the transformer 1.

[0030] The primary circuit 6 is composed of a pair of primary ports p11 and p12, a bypass capacitor, and a full bridge circuit having four switch elements s11, s12, s13, and s14.

[0031] One end of switch element s11 is connected to one primary port p11, and the other end is connected to one end of switch element s12. The other end of switch element s12 is connected to the other primary port p12. One end of switch element s13 is connected to one primary port p11, and the other end is connected to one end of switch element s14. The other end of switch element s14 is connected to the other primary port p12.

[0032] The secondary circuit 7 is composed of a pair of secondary ports p21 and p22, four switch elements s21, s22, s23 and s24, and four bypass capacitors.

[0033] The transformer 1 is provided between a primary circuit 6 and a secondary circuit 7. The transformer 1 is a compound transformer including a primary winding group 31 and a secondary winding group 32.

[0034] The primary winding group 31 is composed of two primary windings 31a and 31b connected in series. One end of the primary winding 31a is connected to a node between switch elements s11 and s12 via a resonant capacitor C11. The other end of the primary winding 31a is connected to one end of the other primary winding 31b. The other end of the other primary winding 31b is connected to a node between switch elements s13 and s14.

[0035] The secondary winding group 32 is composed of two secondary windings 32a and 32b connected in parallel. The two secondary windings 32a and 32b are arranged in one-to-one correspondence with the two primary windings 31a and 31b.

[0036] One end of one secondary winding 32a of the two secondary windings is connected to the other secondary port p22 via a switch element s21. The other end of one secondary winding 32a is connected to the other secondary port p22 via a switch element s22. One secondary winding 32a has a center tap, which is connected to one secondary port p21.

[0037] One end of the other secondary winding 32b of the two secondary windings is connected to the other secondary port p22 via a switch element s23. The other end of the other secondary winding 32b is connected to the other secondary port p22 via a switch element s24. The other secondary winding 32b has a center tap, which is connected to one secondary port p21.

[0038] The circuit shown in FIG. 4 also shows the magnetizing inductances Lma and Lmb, leakage inductances Lra and Lrb of the transformer 1, and the capacitance Cr of the resonant capacitor C11, which are shown as an equivalent circuit.

[0039] The excitation inductance Lma is the excitation inductance of one primary winding 31 a and one secondary winding 32 a, the leakage inductance Lra is the leakage inductance of one primary winding 31 a and one secondary winding 32 a, the excitation inductance Lmb is the excitation inductance of the other primary winding 31 b and the other secondary winding 32 b, and the leakage inductance Lrb is the leakage inductance of the other primary winding 31 b and the other secondary winding 32 b.

[0040] [Configuration of Transformer] A specific configuration of the transformer 1 provided in the LLC converter 5 will be described.

[0041] 5A and 5B are external views of the transformer 1. Fig. 5A shows a perspective view of the transformer 1, and Fig. 5B shows a view of the transformer 1 shown in Fig. 5A with the first base portion 11 of the first core 10a removed.

[0042] The transformer 1 includes a core 10 , a primary winding group 31 , and a secondary winding group 32 .

[0043] 6 is a diagram showing the core 10 of the transformer 1. In this figure, the core 10 is shown disassembled into two halves.

[0044] As shown in Fig. 6, the core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and four legs 21, 22, 23, and 24. The first core 10a is formed by integrally molding the first base 11 and the four legs 21 to 24. The second core 10b is composed of a second base 12.

[0045] That is, the core 10 has a first base portion 11, a second base portion 12, and four legs 21 to 24. The four legs 21 to 24 are connected to the first base portion 11. The first base portion 11, the second base portion 12, and the four legs 21 to 24 are each formed of a magnetic material such as Mn—Zn ferrite.

[0046] The first base portion 11 is plate-shaped. The second base portion 12 is plate-shaped and is provided opposite the first base portion 11. When the core 10 is viewed from a first direction x1, which is a direction in which the first base portion 11 and the second base portion 12 face each other, the first base portion 11 and the second base portion 12 are rectangular and at least partially overlap each other.

[0047] Each of the legs 21 to 24 has a columnar shape with its axial direction aligned with the first direction x1. Each of the legs 21 to 24 is provided between the first base 11 and the second base 12, and is in contact with the second base 12 in the first direction x1 or faces the second base 12 across a gap. The legs 21 to 24 are spaced apart in a second direction x2 that intersects with the first direction x1. The legs 21 to 24 are arranged side by side in order along the second direction x2, which is a linear direction. In this embodiment, the first direction x1 and the second direction x2 are perpendicular to each other.

[0048] Hereinafter, the direction in which the first base 11 and the second base 12 face each other will be referred to as the first direction x1, and the direction in which the legs 21 to 24 are arranged side by side will be referred to as the second direction x2. In addition, the direction perpendicular to both the first direction x1 and the second direction x2 will sometimes be referred to as the third direction x3.

[0049] Of the multiple legs 21 to 24, legs 21 and 24 are located outermost in the second direction x2. The legs 21 and 24 have a rectangular cross section perpendicular to the first direction x1. The legs 21 and 24 are arranged so that the short sides of the rectangles are parallel to an axis along the second direction x2. The first core 10a has a symmetrical shape with respect to a center line located midway between the outermost legs 21 and 24.

[0050] The legs 22 and 23 are located more inward in the second direction x2 than the legs 21 and 24 provided on both outer sides. The legs 22 and 23 have an elliptical cross section perpendicular to the first direction x1. The legs 22 and 23 are arranged so that the width of the elliptical shape is parallel to an axis along the second direction x2. The cross sections of the legs 22 and 23 may be circular, elliptical, polygonal, or oval.

[0051] Although the above example shows all of the legs 21 to 24 being connected to the first base 11, this is not limiting. All of the legs 21 to 24 may be connected to the second base 12 instead of the first base 11. Alternatively, some of the four legs 21 to 24 may be connected to the first base 11, and the remaining legs may be connected to the second base 12. Furthermore, the legs 21 to 24 do not necessarily have to be directly connected to the first base 11 or the second base 12. For example, the legs 21 to 24 may be connected to the first base 11 and the second base 12 via a non-magnetic material portion that does not contain a magnetic material. Furthermore, if the legs 21 to 24 are held by another support member, a gap may be provided between the legs 21 to 24 and the first base 11 or between the legs 21 to 24 and the second base 12.

[0052] Furthermore, the four legs 21 to 24 may be divided into two in the first direction x1. For example, when each of the legs 21 to 24 is divided into two in the first direction x1, one of the divided portions may be connected to the first base 11, and the other portion may be connected to the second base 12. In this case, the legs protrude from both the first base 11 and the second base 12 so as to face each other. A gap may be provided in the opposing region of the two opposing legs.

[0053] Fig. 7 is a schematic diagram of the transformer 1. In Fig. 7, the primary winding group 31 is indicated by narrow hatching, and the secondary winding group 32 is indicated by wide hatching.

[0054] The primary winding group 31 and the secondary winding group 32 are provided, for example, inside or on the surface of a printed circuit board 70 (see FIG. 5). The printed circuit board 70 has four through holes. The legs 21 to 24 of the core 10 are inserted into the four through holes of the printed circuit board 70, respectively. By inserting the legs 21 to 24 into the four through holes, the primary winding group 31 and the secondary winding group 32 are wound around each leg. The actual printed circuit board 70 is larger than the size shown in FIG. 5, and the above-mentioned primary side circuit 6, secondary side circuit 7, etc. are mounted on it.

[0055] The primary winding group 31 and the secondary winding group 32 may be formed by winding wires with insulating coating around the legs, rather than being formed on the printed circuit board 70. In the following schematic diagrams, similar to Fig. 7, the base material of the printed circuit board 70 may be omitted from illustration, and only the primary winding group 31 and the secondary winding group 32 of the printed circuit board 70 may be illustrated.

[0056] The primary winding group 31 includes four or more primary coils. In this example, the primary winding group 31 has four primary coils 41, 42, 43, and 44.

[0057] Of the four legs, two legs 21 and 24 are wound with primary coils 41 and 44, respectively. Of the four legs, two legs 22 and 23 different from the two legs 21 and 24 are wound with primary coils 42 and 43 different from the primary coils 41 and 44, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound on the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0058] As described above, the primary winding group 31 is composed of two primary windings 31a and 31b. Each of the two primary windings 31a and 31b is composed of a primary coil and another primary coil connected in series. Specifically, one primary winding 31a is composed of a primary coil 41 and another primary coil 42 connected in series to the primary coil 41. The other primary winding 31b is composed of a primary coil 44 and another primary coil 43 connected in series to the primary coil 44. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0059] The connection relationship of the primary coils 41 to 44 is not limited to the above, and for example, the primary coil 41 may be connected in series with another primary coil 43, and the primary coil 44 may be connected in series with another primary coil 42. The primary coil 41 may be connected in series with the primary coil 44, and the other primary coil 42 may be connected in series with another primary coil 43. The four primary coils 41 to 44 may be connected in series in any order.

[0060] The primary coils 41 to 44 wound around the legs 21 to 24 have the same number of turns. The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other.

[0061] For example, when the transformer 1 is viewed from the first base 11 toward the second base 12 (the opposite direction of the arrow in the first direction x1), the primary coil 41 wound around the leg 21 is wound clockwise, the other primary coil 42 wound around the leg 22 is wound counterclockwise, the other primary coil 43 wound around the leg 23 is wound clockwise, and the primary coil 44 wound around the leg 24 is wound counterclockwise. In this way, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2. Note that the above clockwise and counterclockwise relationships may be reversed.

[0062] The secondary winding group 32 includes two or more secondary coils. In this example, the secondary winding group 32 has four secondary coils 51, 52, 53, and 54.

[0063] As described above, the secondary winding group 32 is composed of two secondary windings 32a and 32b. Each of the two secondary windings 32a and 32b is composed of two secondary coils connected in parallel. Specifically, one secondary winding 32a is composed of two secondary coils 51 and 52 connected in parallel, and the other secondary winding 32b is composed of two secondary coils 53 and 54 connected in parallel. Since the secondary windings 32a and 32b are connected in parallel, these four secondary coils 51 to 54 are connected in parallel with each other.

[0064] Of the four legs 21 to 24, two legs 21 and 24 are wound with secondary coils in addition to the primary coils described above. For example, two secondary coils 51 and 52 are wound around leg 21, and two secondary coils 53 and 54 are wound around leg 24. In contrast, no secondary coils are wound around the other two legs 22 and 23, and only the other primary coils 42 and 43 are wound around them. Note that in this example, the number of turns in the secondary coil is less than the number of turns in the primary coil.

[0065] The secondary coils 51 to 54 in FIG. 7 have one turn or less, so the winding direction does not need to be specified. However, if the number of turns is one turn or more, the winding directions may be as follows. For example, the two secondary coils 51, 52 wound around the leg 21 may have the same winding direction. The two secondary coils 53, 54 wound around the leg 24 may have the same winding direction. The winding directions of the secondary coils 51, 52 wound around the leg 21 and the secondary coils 53, 54 wound around the leg 24 may be opposite. Furthermore, the winding directions of the secondary coils 51, 52 wound around the leg 21 and the primary coil 41 may be the same. The winding directions of the secondary coils 53, 54 wound around the leg 24 and the primary coil 44 may be the same.

[0066] FIG. 8 is a diagram showing magnetic flux generated in the transformer 1.

[0067] In the figure, the magnetic flux generated in the core 10 when a current flows through the primary winding group 31 is indicated by arrows in the positive half cycle of the AC voltage waveform. Note that the arrows point in the opposite direction in the negative half cycle.

[0068] The magnetic flux φa1 is a magnetic flux generated in the core 10 by the primary coil 41 and the other primary coil 42. The magnetic flux φa1 is formed in a loop-shaped first magnetic circuit mc1 that includes the leg 21, a part of the second base 12, the leg 22, and a part of the first base 11. The primary winding group 31 is wound around the two legs 21 and 22 so that the magnetic fluxes generated in the two legs 21 and 22 reinforce each other. The magnetic flux φa1 generated by the primary winding group 31 interlinks with one of the secondary windings 32a (i.e., the secondary coils 51 and 52) at the leg 21, but does not interlink with the secondary winding group 32 at the leg 22.

[0069] The magnetic flux φb is magnetic flux generated in the core 10 by the other primary coil 42 and the other primary coil 43. The magnetic flux φb is formed in a loop-shaped second magnetic circuit mc2 that includes the leg 22, a part of the first base 11, the leg 23, and a part of the second base 12. The primary winding group 31 is wound around the two legs 22 and 23 so that the magnetic fluxes generated in the two legs 22 and 23 reinforce each other. The magnetic flux φb generated by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 22 and 23.

[0070] The magnetic flux φa2 is a magnetic flux generated in the core 10 by the other primary coils 43 and 44. The magnetic flux φa2 is formed in a loop-shaped third magnetic circuit mc3 that includes the leg 23, a part of the second base 12, the leg 24, and a part of the first base 11. The primary winding group 31 is wound around the two legs 23 and 24 so that the magnetic fluxes generated in the two legs 23 and 24 reinforce each other. The magnetic flux φa2 generated by the primary winding group 31 does not interlink with the secondary winding group 32 at the leg 23, but interlinks with the other secondary winding 32b (i.e., the secondary coils 53 and 54) at the leg 24.

[0071] In the transformer 1 of this embodiment, when a current flows through the primary winding group 31, a magnetic flux is generated in each of the four legs 21 to 24. The primary winding group 31 is wound around the four legs 21 to 24 so that the magnetic fluxes generated in two legs adjacent to each other in the second direction x2 are reinforced. That is, in the transformer 1, a current flows through the primary winding group 31 in a direction that reinforces the magnetic fluxes generated in two legs adjacent to each other in the second direction x2.

[0072] Furthermore, in the transformer 1 of this embodiment, the magnetic flux φa1 generated by the primary winding group 31 is configured to interlink with the secondary winding 32a wound around the leg 21. This allows magnetic fluxes of different magnitudes to be generated in the first magnetic circuit mc1 including the legs 21 and 22 and the second magnetic circuit mc2 including the legs 22 and 23. Furthermore, in the transformer 1, the magnetic flux φa2 generated by the primary winding group 31 is configured to interlink with the secondary winding 32b wound around the leg 24. This allows magnetic fluxes of different magnitudes to be generated in the second magnetic circuit mc2 including the legs 22 and 23 and the third magnetic circuit mc3 including the legs 23 and 24. With this transformer 1, magnetic fluxes of different magnitudes can be generated in multiple regions, thereby increasing leakage inductance.

[0073] FIG. 9 is a diagram showing an example of the gain characteristic of the LLC converter 5 using the transformer 1.

[0074] The horizontal axis of the figure shows normalized frequency, and the vertical axis shows the transfer function, which is gain. In the figure, Q is the ratio of impedance Zr to primary-converted equivalent resistance Rac, and is calculated by Q=Zr / Rac (Equation 2). Note that impedance Zr is the ratio of leakage inductance Lr to capacitance Cr of resonance capacitor C11, and is calculated by Zr=√(Lr / Cr). Rac is the primary-converted equivalent resistance when the LLC converter is represented by an equivalent circuit, and Rac=8N ２ R / π ２ N is the turn ratio between the primary winding group 31 and the secondary winding group 32, and R is the load resistance.

[0075] 9, in the LLC converter 5 using the transformer 1, it is possible to adjust the gain range by adjusting the Q factor. For example, the smaller the Q factor, the greater the change in gain can be.

[0076] In the power supply for on-board accessories (12 V), the current on the secondary side is large, but in this embodiment, the secondary windings 32 a, 32 b on the secondary side are connected in parallel. For example, in the comparative example, there is only one secondary winding on the secondary side, which results in a large current value per phase and a large Q value. However, in the LLC converter 5 of this embodiment, there are two secondary windings on the secondary side, which allows the current value and Q value per phase to be reduced. In this way, the LLC converter 5 of this embodiment can reduce the Q value, thereby enabling a wide gain range. Furthermore, in the LLC converter 5, the current value per phase can be reduced, which allows for reduced conduction loss.

[0077] [Modification 1] A transformer 1A according to Modification 1 of the embodiment will be described. In Modification 1, an example will be described in which secondary coils 51 to 54 are provided on inner legs 22 and 23 rather than on outermost legs 21 and 24.

[0078] FIG. 10 is a schematic diagram of a transformer 1A according to a first modification of the embodiment.

[0079] 10, the transformer 1A includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 has the same configuration as in the embodiment.

[0080] 10 includes four or more primary coils. The primary winding group 31 has four primary coils 41, 42, 43, and 44.

[0081] Of the four legs, two legs 22 and 23 are wound with primary coils 42 and 43, respectively. Of the four legs, two legs 21 and 24 different from the two legs 22 and 23 are wound with primary coils 41 and 44 different from the primary coils 42 and 43, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound on the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0082] The primary winding group 31 is composed of two primary windings 31a and 31b. Each of the two primary windings 31a and 31b is composed of a primary coil and another primary coil connected in series. Specifically, one primary winding 31a is composed of a primary coil 42 and another primary coil 41 connected in series to the primary coil 42. The other primary winding 31b is composed of a primary coil 43 and another primary coil 44 connected in series to the primary coil 43. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0083] The primary coils 41 to 44 wound around the legs 21 to 24 have the same number of turns. The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other.

[0084] For example, when the transformer 1A is viewed from the first base 11 toward the second base 12 (the opposite direction of the arrow in the first direction x1), the other primary coil 41 wound around the leg 21 is clockwise, the primary coil 42 wound around the leg 22 is counterclockwise, the primary coil 43 wound around the leg 23 is clockwise, and the other primary coil 44 wound around the leg 24 is counterclockwise. In this way, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2. Note that the above clockwise and counterclockwise relationships may be reversed.

[0085] 10 includes two or more secondary coils. The secondary winding group 32 has four secondary coils 51, 52, 53, and 54.

[0086] The secondary winding group 32 is composed of two secondary windings 32a and 32b. Each of the two secondary windings 32a and 32b is composed of two secondary coils connected in parallel. Specifically, one secondary winding 32a is composed of two secondary coils 51 and 52 connected in parallel, and the other secondary winding 32b is composed of two secondary coils 53 and 54 connected in parallel. Since the secondary windings 32a and 32b are connected in parallel, these four secondary coils 51 to 54 are connected in parallel with each other.

[0087] Of the four legs 21 to 24, two legs 22 and 23 are wound with secondary coils in addition to the primary coils described above. For example, two secondary coils 51 and 52 are wound around leg 22, and two secondary coils 53 and 54 are wound around leg 23. In contrast, no secondary coils are wound around the other two legs 21 and 24, and only the other primary coils 41 and 44 are wound around them. Note that in this example, the number of turns in the secondary coil is less than the number of turns in the primary coil.

[0088] The secondary coils 51 to 54 in FIG. 10 have one turn or less, so the winding direction does not need to be specified. However, if the number of turns is one turn or more, the winding directions may be as follows. For example, the two secondary coils 51, 52 wound around the leg 22 may have the same winding direction. The two secondary coils 53, 54 wound around the leg 23 may have the same winding direction. The winding directions of the secondary coils 51, 52 wound around the leg 22 and the secondary coils 53, 54 wound around the leg 23 may be opposite. Furthermore, the winding directions of the secondary coils 51, 52 wound around the leg 22 and the primary coil 42 may be the same. The winding directions of the secondary coils 53, 54 wound around the leg 23 and the primary coil 43 may be the same.

[0089] FIG. 10 also shows the magnetic flux generated in the transformer 1A.

[0090] The magnetic flux φa1 is a magnetic flux generated in the core 10 by the other primary coil 41 and primary coil 42. The magnetic flux φa1 is formed in a loop-shaped first magnetic circuit mc1 that includes the leg 21, a part of the second base 12, the leg 22, and a part of the first base 11. The primary winding group 31 is wound around the two legs 21 and 22 so that the magnetic fluxes generated in the two legs 21 and 22 reinforce each other. The magnetic flux φa1 generated by the primary winding group 31 does not interlink with the secondary winding group 32 at the leg 21, but interlinks with one of the secondary windings 32a (i.e., the secondary coils 51 and 52) at the leg 22.

[0091] The magnetic flux φa is a magnetic flux generated in the core 10 by the primary coil 42 and the primary coil 43. The magnetic flux φa is formed in a loop-shaped second magnetic circuit mc2 that includes the leg 22, a portion of the first base 11, the leg 23, and a portion of the second base 12. The primary winding group 31 is wound around the two legs 22 and 23 so that the magnetic fluxes generated in the two legs 22 and 23 reinforce each other. The magnetic flux φa generated by the primary winding group 31 interlinks with one secondary winding 32a at the leg 22 and with the other secondary winding 32b at the leg 23. In other words, the magnetic flux φa interlinks with the secondary winding group 32 approximately evenly at both the legs 22 and 23.

[0092] The magnetic flux φa2 is generated in the core 10 by the primary coil 43 and the other primary coil 44. The magnetic flux φa2 is formed in a loop-shaped third magnetic circuit mc3 that includes the leg 23, a part of the second base 12, the leg 24, and a part of the first base 11. The primary winding group 31 is wound around the two legs 23, 24 so that the magnetic fluxes generated in the two legs 23, 24 reinforce each other. The magnetic flux φa2 generated by the primary winding group 31 interlinks with the other secondary winding 32b (i.e., the secondary coils 53, 54) at the leg 23, but does not interlink with the secondary winding group 32 at the leg 24.

[0093] In the transformer 1A of the first modification, when a current flows through the primary winding group 31, a magnetic flux is generated in each of the four legs 21 to 24. The primary winding group 31 is wound around the four legs 21 to 24 so that the magnetic fluxes generated in two legs adjacent to each other in the second direction x2 are reinforced. That is, in the transformer 1A, a current flows through the primary winding group 31 in a direction that reinforces the magnetic fluxes generated in two legs adjacent to each other in the second direction x2.

[0094] Furthermore, in the transformer 1A of the first modification, the magnetic flux φa1 generated by the primary winding group 31 is configured to interlink with the secondary winding 32a wound around the leg 22. This allows magnetic fluxes of different magnitudes to be generated in the first magnetic circuit mc1 including the legs 21 and 22 and the second magnetic circuit mc2 including the legs 22 and 23. Furthermore, in the transformer 1A, the magnetic flux φa2 generated by the primary winding group 31 is configured to interlink with the secondary winding 32b wound around the leg 23. This allows magnetic fluxes of different magnitudes to be generated in the second magnetic circuit mc2 including the legs 22 and 23 and the third magnetic circuit mc3 including the legs 23 and 24. With this transformer 1A, magnetic fluxes of different magnitudes can be generated in multiple regions, thereby increasing leakage inductance.

[0095] [Modification 2] A transformer 1B according to Modification 2 of the embodiment will be described. In Modification 2, an example in which the core 10 has six legs will be described.

[0096] FIG. 11 is a schematic diagram of a transformer 1B according to a second modification of the embodiment.

[0097] As shown in FIG. 11, the transformer 1B includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0098] The core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and six legs 21, 22, 23, 24, 25, and 26. The second core 10b is composed of a second base 12. In other words, the core 10 has the first base 11, the second base 12, and the six legs 21 to 26. The six legs 21 to 26 are connected to the first base 11.

[0099] Each of the legs 21 to 26 has a columnar shape with its axial direction aligned with the first direction x1. Each of the legs 21 to 26 is provided between the first base 11 and the second base 12, and is in contact with the second base 12 in the first direction x1 or faces the second base 12 across a gap. The legs 21 to 26 are spaced apart in the second direction x2. In this modified example, the legs 21 to 26 are arranged side by side in order along the second direction x2, which is perpendicular to the first direction x1.

[0100] The legs 21 and 26 are located at the outermost positions in the second direction x2. The first core 10a has a shape symmetrical with respect to a center line located midway between the outermost legs 21 and 26. The legs 22 to 25 are located more inward than the outermost legs 21 and 26 in the second direction x2.

[0101] The primary winding group 31 shown in FIG. 11 has six primary coils 41, 42, 43, 44, 45 and 46.

[0102] Of the six legs, two legs 21 and 26 are wound with primary coils 41 and 46, respectively. Of the six legs, four legs 22, 23, 24 and 25 other than the two legs 21 and 26 are wound with primary coils 42, 43, 44 and 45 other than the primary coils 41 and 46, respectively. In other words, the primary coils 41, 42, 43, 44, 45 and 46 are wound on the legs 21, 22, 23, 24, 25 and 26 in one-to-one correspondence.

[0103] The primary winding group 31 is composed of two primary windings 31a and 31b. Each of the two primary windings 31a and 31b is composed of a primary coil and another primary coil connected in series. Specifically, one primary winding 31a is composed of a primary coil 41, another primary coil 42 connected to the primary coil 41, and another primary coil 43 connected to the other primary coil 42. The other primary winding 31b is composed of a primary coil 46, another primary coil 45 connected to the primary coil 46, and another primary coil 44 connected to the other primary coil 45. Since the primary windings 31a and 31b are connected in series, these six primary coils 41 to 46 are also connected in series.

[0104] The primary coils 41 to 46 wound around the legs 21 to 26 have the same number of turns. The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other.

[0105] For example, when transformer 1B is viewed from first base 11 to second base 12, primary coil 41 wound around leg 21 is wound clockwise, another primary coil 42 wound around leg 22 is wound counterclockwise, another primary coil 43 wound around leg 23 is wound clockwise, another primary coil 44 wound around leg 24 is wound counterclockwise, another primary coil 45 wound around leg 25 is wound clockwise, and primary coil 46 wound around leg 26 is wound counterclockwise. In this way, primary coils 41 to 46 are wound in alternate opposite directions in the order in which they are lined up in second direction x2.

[0106] 11 includes two or more secondary coils. In this example, the secondary winding group 32 has two secondary coils 51 and 53.

[0107] The secondary winding group 32 is composed of two secondary windings 32a and 32b. In this example, the secondary winding 32a corresponds to the secondary coil 51, and the secondary winding 32b corresponds to the secondary coil 53. The secondary windings 32a and 32b, i.e., the secondary coils 51 and 53, are connected in parallel with each other.

[0108] In addition to the primary coil described above, secondary coils are also wound around two of the six legs 21 to 26. For example, secondary coil 51 is wound around leg 21, and secondary coil 53 is wound around leg 26. In contrast, no secondary coils are wound around the other four legs 22 to 25, and only the other primary coils 42 to 45 are wound around them.

[0109] FIG. 11 also shows the magnetic flux generated in the transformer 1B.

[0110] The magnetic flux φa1 is a magnetic flux generated in the core 10 by the primary coil 41 and the other primary coil 42. The magnetic flux φa1 is formed in a loop-shaped first magnetic circuit mc1 that includes the leg 21, a part of the second base 12, the leg 22, and a part of the first base 11. The primary winding group 31 is wound around the two legs 21 and 22 so that the magnetic fluxes generated in the two legs 21 and 22 reinforce each other. The magnetic flux φa1 generated by the primary winding group 31 interlinks with one secondary winding 32a (i.e., the secondary coil 51) at the leg 21 and does not interlink with the secondary winding group 32 at the leg 22.

[0111] The magnetic flux φb1 is a magnetic flux generated in the core 10 by the other primary coil 42 and the other primary coil 43. The magnetic flux φb1 is formed in a loop-shaped second magnetic circuit mc2 that includes the leg 22, a part of the first base 11, the leg 23, and a part of the second base 12. The primary winding group 31 is wound around the two legs 22 and 23 so that the magnetic fluxes generated in the two legs 22 and 23 reinforce each other. The magnetic flux φb1 generated by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 22 and 23.

[0112] The magnetic flux φb2 is a magnetic flux generated in the core 10 by the other primary coil 43 and the other primary coil 44. The magnetic flux φb2 is formed in a loop-shaped third magnetic circuit mc3 that includes the leg 23, a part of the second base 12, the leg 24, and a part of the first base 11. The primary winding group 31 is wound around the two legs 23, 24 so that the magnetic fluxes generated in the two legs 23, 24 reinforce each other. The magnetic flux φb2 generated by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 23, 24.

[0113] The magnetic flux φb3 is generated in the core 10 by the other primary coils 44 and 45. The magnetic flux φb3 is formed in a loop-shaped fourth magnetic circuit mc4 that includes the leg 24, a part of the first base 11, the leg 25, and a part of the second base 12. The primary winding group 31 is wound around the two legs 24 and 25 so that the magnetic fluxes generated in the two legs 24 and 25 reinforce each other. The magnetic flux φb3 controlled by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 24 and 25.

[0114] The magnetic flux φa2 is a magnetic flux generated in the core 10 by the other primary coils 45 and 46. The magnetic flux φa2 is formed in a loop-shaped fifth magnetic circuit mc5 that includes the leg 25, a part of the second base 12, the leg 26, and a part of the first base 11. The primary winding group 31 is wound around the two legs 25 and 26 so that the magnetic fluxes generated in the two legs 25 and 26 reinforce each other. The magnetic flux φa2 generated by the primary winding group 31 does not interlink with the secondary winding group 32 at the leg 25, but interlinks with the other secondary winding 32b (i.e., the secondary coil 53) at the leg 26.

[0115] In the transformer 1B of the second modification, when a current flows through the primary winding group 31, a magnetic flux is generated in each of the six legs 21 to 26. The primary winding group 31 is wound around the six legs 21 to 26 so that the magnetic fluxes generated in two legs adjacent to each other in the second direction x2 are reinforced. That is, in the transformer 1B, a current flows through the primary winding group 31 in a direction that reinforces the magnetic fluxes generated in two legs adjacent to each other in the second direction x2.

[0116] Furthermore, in the transformer 1B of the second modification, the magnetic flux φa1 generated by the primary winding group 31 is configured to interlink with the secondary winding 32a wound around the leg 21. This allows magnetic fluxes of different magnitudes to be generated in the first magnetic circuit mc1 including the legs 21 and 22 and the second magnetic circuit mc2 including the legs 22 and 23. Furthermore, in the transformer 1B, the magnetic flux φa2 generated by the primary winding group 31 is configured to interlink with the secondary winding 32b wound around the leg 26. This allows magnetic fluxes of different magnitudes to be generated in the fourth magnetic circuit mc4 including the legs 24 and 25 and the fifth magnetic circuit mc5 including the legs 25 and 26. With this transformer 1B, magnetic fluxes of different magnitudes can be generated in multiple regions, thereby increasing leakage inductance.

[0117] [Modification 3] A transformer 1C according to Modification 3 of the embodiment will be described. In Modification 3, an example in which the core 10 has seven legs will be described.

[0118] FIG. 12 is a schematic diagram of a transformer 1C according to a third modification of the embodiment.

[0119] As shown in FIG. 12, a transformer 1C includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0120] The core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and seven legs 21, 22, 23, 24, 25, 26, and 27. The second core 10b is composed of a second base 12. In other words, the core 10 has the first base 11, the second base 12, and seven legs 21 to 27. The seven legs 21 to 27 are connected to the first base 11.

[0121] Each of the legs 21 to 27 has a columnar shape with its axial direction aligned with the first direction x1. Each of the legs 21 to 27 is provided between the first base 11 and the second base 12, and is in contact with the second base 12 in the first direction x1 or faces the second base 12 across a gap. The legs 21 to 27 are spaced apart from each other in the second direction x2. In this modified example, the legs 21 to 27 are arranged side by side in order along the second direction x2, which is perpendicular to the first direction x1.

[0122] The legs 21 and 27 are located at the outermost sides in the second direction x2. The leg 24 is located at the center of the multiple legs 21 to 27 in the second direction x2. The first core 10a has a shape that is symmetrical with respect to a center line located halfway between the outermost legs 21 and 27. The legs 22, 23, 25, and 26 are located more inward than the legs 21 and 27 located on both outer sides in the second direction x2. The legs 22 and 23 are located between the legs 21 and 24, and the legs 25 and 26 are located between the legs 24 and 27.

[0123] The primary winding group 31 shown in FIG. 12 has seven primary coils 41, 42, 43, 44, 45, 46 and 47.

[0124] Of the seven legs, three legs 21, 24, and 27 are wound with primary coils 41, 44, and 47, respectively. Of the seven legs, four legs 22, 23, 25, and 26 other than the three legs 21, 24, and 27 are wound with primary coils 42, 43, 45, and 46 other than the primary coils 41, 44, and 47, respectively. In other words, the primary coils 41, 42, 43, 44, 45, 46, and 47 are wound on the legs 21, 22, 23, 24, 25, 26, and 27 in one-to-one correspondence.

[0125] The primary winding group 31 is made up of a plurality of primary windings. Each of the plurality of primary windings is made up of a primary coil and another primary coil connected in series. The plurality of primary windings are connected in series, and seven primary coils 41 to 47 are connected in series.

[0126] The primary coils 41 to 47 wound around the legs 21 to 27 have the same number of turns. The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other.

[0127] For example, when the transformer 1C is viewed from the first base 11 toward the second base 12, the primary coil 41 wound around the leg 21 is wound clockwise, the other primary coil 42 wound around the leg 22 is wound counterclockwise, the other primary coil 43 wound around the leg 23 is wound clockwise, the primary coil 44 wound around the leg 24 is wound counterclockwise, the other primary coil 45 wound around the leg 25 is wound clockwise, the other primary coil 46 wound around the leg 26 is wound counterclockwise, and the primary coil 47 wound around the leg 27 is wound clockwise. In this way, the primary coils 41 to 47 are wound in alternate opposite directions in the order in which they are lined up in the second direction x2.

[0128] The secondary winding group 32 shown in FIG. 12 includes a plurality of secondary coils 51 , 53 , and 55 .

[0129] The secondary winding group 32 is made up of a plurality of secondary windings. The plurality of secondary windings is made up of a plurality of secondary coils 51, 53, and 55. The plurality of secondary windings, i.e., the plurality of secondary coils 51, 53, and 55, are connected in parallel with one another.

[0130] Of the seven legs 21 to 27, three legs 21, 24, and 27 are wound with secondary coils in addition to the primary coils described above. For example, secondary coil 51 is wound around leg 21, secondary coil 55 is wound around leg 24, and secondary coil 53 is wound around leg 27. In contrast, no secondary coils are wound around the other four legs 22, 23, 25, and 26, and only the other primary coils 42, 43, 45, and 46 are wound around them.

[0131] FIG. 12 also shows the magnetic flux generated in the transformer 1C.

[0132] The magnetic flux φa1 is a magnetic flux generated in the core 10 by the primary coil 41 and the other primary coils 42. The magnetic flux φa1 is formed in a loop-shaped first magnetic circuit mc1 that includes the leg 21, a portion of the first base 11, the leg 22, and a portion of the second base 12. The primary winding group 31 is wound around the two legs 21 and 22 so that the magnetic fluxes generated in the two legs 21 and 22 reinforce each other. The magnetic flux φa1 generated by the primary winding group 31 interlinks with the secondary winding group 32 (in this case, the secondary coil 51) at the leg 21, but does not interlink with the secondary winding group 32 at the leg 22.

[0133] The magnetic flux φb1 is a magnetic flux generated in the core 10 by the other primary coil 42 and the other primary coil 43. The magnetic flux φb1 is formed in a loop-shaped second magnetic circuit mc2 that includes the leg 22, a part of the second base 12, the leg 23, and a part of the first base 11. The primary winding group 31 is wound around the two legs 22 and 23 so that the magnetic fluxes generated in the two legs 22 and 23 reinforce each other. The magnetic flux φb1 generated by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 22 and 23.

[0134] The magnetic flux φa2 is a magnetic flux generated in the core 10 by the other primary coils 43 and 44. The magnetic flux φa2 is formed in a loop-shaped third magnetic circuit mc3 that includes the leg 23, a portion of the first base 11, the leg 24, and a portion of the second base 12. The primary winding group 31 is wound around the two legs 23 and 24 so that the magnetic fluxes generated in the two legs 23 and 24 reinforce each other. The magnetic flux φa2 generated by the primary winding group 31 does not interlink with the secondary winding group 32 at the leg 23, but interlinks with the secondary winding group 32 (in this case, the secondary coil 55) at the leg 24.

[0135] The magnetic flux φa3 is a magnetic flux generated in the core 10 by the primary coil 44 and the other primary coils 45. The magnetic flux φa3 is formed in a loop-shaped fourth magnetic circuit mc4 that includes the leg 24, a part of the second base 12, the leg 25, and a part of the first base 11. The primary winding group 31 is wound around the two legs 24, 25 so that the magnetic fluxes generated in the two legs 24, 25 reinforce each other. The magnetic flux φa3 generated by the primary winding group 31 interlinks with the secondary winding group 32 (in this case, the secondary coil 55) at the leg 24, but does not interlink with the secondary winding group 32 at the leg 24.

[0136] The magnetic flux φb2 is a magnetic flux generated in the core 10 by the other primary coil 45 and the other primary coil 46. The magnetic flux φb2 is formed in a loop-shaped fifth magnetic circuit mc5 that includes the leg 25, a part of the first base 11, the leg 26, and a part of the second base 12. The primary winding group 31 is wound around the two legs 25, 26 so that the magnetic fluxes generated in the two legs 25, 26 reinforce each other. The magnetic flux φb2 generated by the primary winding group 31 does not interlink with the secondary winding group 32 in either of the legs 25, 26.

[0137] The magnetic flux φa4 is a magnetic flux generated in the core 10 by the other primary coils 46 and 47. The magnetic flux φa4 is formed in a loop-shaped sixth magnetic circuit mc6 that includes the leg 26, a portion of the second base 12, the leg 27, and a portion of the first base 11. The primary winding group 31 is wound around the two legs 26 and 27 so that the magnetic fluxes generated in the two legs 26 and 27 reinforce each other. The magnetic flux φa4 generated by the primary winding group 31 does not interlink with the secondary winding group 32 at the leg 26, but interlinks with the secondary winding group 32 (in this case, the secondary coil 53) at the leg 27.

[0138] In the transformer 1C of the third modification, when a current flows through the primary winding group 31, a magnetic flux is generated in each of the seven legs 21 to 27. The primary winding group 31 is wound around the seven legs 21 to 27 so that the magnetic fluxes generated in two legs adjacent to each other in the second direction x2 are reinforced. That is, in the transformer 1C, a current flows through the primary winding group 31 in a direction that reinforces the magnetic fluxes generated in two legs adjacent to each other in the second direction x2.

[0139] Furthermore, in the transformer 1C of the third modification, the magnetic flux φa1 generated by the primary winding group 31 is configured to interlink with the secondary winding group 32 wound around the leg 21. This allows magnetic fluxes of different magnitudes to be generated in the first magnetic circuit mc1 including the legs 21 and 22 and the second magnetic circuit mc2 including the legs 22 and 23. Furthermore, in the transformer 1C, the magnetic flux φa2 generated by the primary winding group 31 is configured to interlink with the secondary winding group 32 wound around the leg 24. This allows magnetic fluxes of different magnitudes to be generated in the second magnetic circuit mc2 including the legs 22 and 23 and the third magnetic circuit mc3 including the legs 23 and 24. Furthermore, in the transformer 1C, the magnetic flux φa3 generated by the primary winding group 31 is configured to interlink with the secondary winding group 32 wound around the leg 24. This allows magnetic fluxes of different magnitudes to be generated in the fourth magnetic circuit mc4 including legs 24 and 25 and the fifth magnetic circuit mc5 including legs 25 and 26. Furthermore, in transformer 1C, magnetic flux φa4 generated by primary winding group 31 is configured to interlink with secondary winding group 32 wound around leg 27. This allows magnetic fluxes of different magnitudes to be generated in the fifth magnetic circuit mc5 including legs 25 and 26 and the sixth magnetic circuit mc6 including legs 26 and 27. With this transformer 1C, magnetic fluxes of different magnitudes can be generated in multiple regions, thereby increasing leakage inductance.

[0140] [Modification 4] A transformer 1D according to Modification 4 of the embodiment will be described. Modification 4 describes an example in which the gap between the leg portions and the base portion of the core 10 varies depending on the leg portions.

[0141] The transformer 1D of the fourth modification includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0142] FIG. 13A is a schematic diagram illustrating an example of a transformer 1D according to a fourth modification of the embodiment.

[0143] As shown in Fig. 13A, the core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and four legs 21, 22, 23, and 24. The four legs 21 to 24 are arranged side by side along the second direction x2. Each of the legs 21 to 24 is provided between the first base 11 and the second base 12, and is in contact with the second base 12 in the first direction x1 or faces the second base 12 across a gap g.

[0144] For example, by providing a gap g between the legs 21 to 24 of the core 10 and the second base portion 12, it is possible to reduce the saturation magnetic flux density in the core 10. In the above, an example has been shown in which the transformer 1D has four legs 21 to 24, but this is not limiting, and the number of legs may be four or more.

[0145] In addition, in the example shown in Figure 13A, the distance Ia in the first direction x1 between the legs 21, 24 located outermost in the second direction x2 and the second base 12 is narrower than the distance Ib in the first direction x1 between the legs 22, 23 located inner than the outermost and the second base 12.

[0146] Fig. 13B is a schematic diagram showing another example of a transformer 1D according to Modification 4. In Fig. 13B, the gap Ia is wider than the gap Ib.

[0147] Although the example above shows a case where a gap is provided between the four legs 21 to 24 and the second base 12, the present invention is not limited to this, and a gap may be provided between the four legs 21 to 24 and the first base 11. Note that the gap between the legs and the base may be zero, and the legs and the base may be in contact with each other.

[0148] In the fourth modification, the distance Ia in the first direction x1 between the leg 21, 24 located outermost in the second direction x2 among the four leg portions 21 to 24 and the first base portion 11 or the second base portion 12 is different from the distance Ib in the first direction x1 between the leg 22, 23 located more inward than the outermost and the first base portion 11 or the second base portion 12. By making the distance Ia and the distance Ib different in this way, a desired magnitude of leakage inductance can be obtained.

[0149] [Modification 5] A transformer 1E according to Modification 5 of the embodiment will be described. In Modification 5, an example in which the cross-sectional areas of the legs and base of core 10 are different will be described.

[0150] The transformer 1E of the fifth modification includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0151] FIG. 14 is a diagram illustrating a core 10 of a transformer 1E according to a fifth modification of the embodiment.

[0152] As shown in Fig. 14, the core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and four legs 21, 22, 23, and 24. The four legs 21 to 24 are arranged side by side along the second direction x2. The number of legs may be four or more.

[0153] 14, the cross-sectional area of ​​the legs 21 and 24 that are located outermost in the second direction x2 among the four legs 21 to 24 is different from the cross-sectional area of ​​the legs 22 and 23 that are located more inward than the outermost, and the cross-sectional area of ​​the first base 11. In the example shown in FIG. 14, the cross-sectional area of ​​the leg 22 or 23 is larger than the cross-sectional area of ​​the leg 21 or 24 and smaller than the cross-sectional area of ​​the first base 11.

[0154] The cross-sectional area is the cross-sectional area of ​​the core 10, and is the area of ​​a plane perpendicular to the magnetic flux that forms the magnetic path. For example, the cross-sectional area of ​​the leg is the area when the leg is cut along a plane perpendicular to the first direction x1. The cross-sectional area of ​​the base is the area when the base is cut along a plane perpendicular to the second direction x2.

[0155] Although the above example shows that the cross-sectional areas of the legs 21 to 24 and the first base 11 are different, this is not limiting. For example, the cross-sectional area may be different only for the second base 12. The relationship in size between the cross-sectional areas of the legs 21 to 24, the first base 11, and the second base 12 may be reversed.

[0156] That is, in transformer 1E of modification 5, at least one of the cross-sectional areas of legs 21 and 24, the cross-sectional area of ​​legs 22 and 23, the cross-sectional area of ​​the plane perpendicular to second direction x2 of first base portion 11, and the cross-sectional area of ​​the plane perpendicular to second direction x2 of second base portion 12 is different. In this way, by partially varying the cross-sectional area of ​​core 10, a desired leakage inductance can be obtained.

[0157] [Modification 6] A transformer 1F according to Modification 6 of the embodiment will be described. Modification 6 describes an example in which the number of turns of one primary coil is different from the number of turns of the other primary coil.

[0158] FIG. 15A is a schematic diagram illustrating an example of a transformer 1F according to a sixth modification of the embodiment.

[0159] 15A includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 and the secondary winding group 32 have the same configurations as those in the embodiment.

[0160] The primary winding group 31 includes four or more primary coils. In this example, the primary winding group 31 has four primary coils 41, 42, 43, and 44. Note that the number of primary coils and legs may be four or more.

[0161] Of the four legs, primary coils 41 and 44 are wound around two legs 21 and 24, respectively. Primary coils 42 and 43 are wound around the other two legs 22 and 23, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound around the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0162] The primary winding group 31 is composed of two primary windings 31a and 31b. One primary winding 31a is composed of a primary coil 41 and another primary coil 42 connected to the primary coil 41. The other primary winding 31b is composed of a primary coil 44 and another primary coil 43 connected to the primary coil 44. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0163] The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other. That is, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2.

[0164] 15A, the number of turns of the primary coil 41 in the leg 21 is smaller than the number of turns of the other primary coils 42 in the leg 22. Also, the number of turns of the primary coil 44 in the leg 24 is smaller than the number of turns of the other primary coils 43 in the leg 23. This makes it possible to reduce the magnetic flux generated by the primary winding group 31 that interlinks with the secondary winding group 32 in each of the legs 21 and 24, and to increase the leakage inductance compared to the embodiment.

[0165] FIG. 15B is a schematic diagram showing another example of a transformer 1F according to the sixth modification of the embodiment.

[0166] 15B includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 and the secondary winding group 32 have the same configurations as those in the first modification.

[0167] The primary winding group 31 includes four or more primary coils. In this example, the primary winding group 31 has four primary coils 41, 42, 43, and 44. Note that the number of primary coils and legs may be four or more.

[0168] Of the four legs, primary coils 42 and 43 are wound around two legs 22 and 23, respectively. Primary coils 41 and 44 are wound around the other two legs 21 and 24, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound around the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0169] The primary winding group 31 is composed of two primary windings 31a and 31b. One primary winding 31a is composed of a primary coil 42 and another primary coil 41 connected to the primary coil 42. The other primary winding 31b is composed of a primary coil 43 and another primary coil 44 connected to the primary coil 43. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0170] The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other. That is, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2.

[0171] 15B, the number of turns of the primary coil 42 in the leg 22 is smaller than the number of turns of the other primary coils 41 in the leg 21. Furthermore, the number of turns of the primary coil 43 in the leg 23 is smaller than the number of turns of the other primary coils 44 in the leg 24. This makes it possible to reduce the magnetic flux generated by the primary winding group 31 that interlinks with the secondary winding group 32 in each of the legs 22 and 23, and to increase the leakage inductance compared to the first modification.

[0172] In the transformer 1F of the sixth modification, the number of turns of the primary coil in the leg around which the primary coil and the secondary coil are wound is different from the number of turns of the primary coil in the leg around which the other primary coil is wound. This allows the magnitude of the magnetic flux generated by the primary winding group 31 that interlinks with the secondary winding group 32 to be changed by the number of turns of the winding, thereby achieving a desired leakage inductance.

[0173] [Variation 7] A transformer 1G according to Variation 7 of the embodiment will be described. Variation 7 describes an example in which the number of turns of one primary coil is greater than the number of turns of the other primary coils.

[0174] FIG. 16A is a schematic diagram illustrating an example of a transformer 1G according to a seventh modification of the embodiment.

[0175] 16A includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 and the secondary winding group 32 have the same configurations as those in the embodiment.

[0176] The primary winding group 31 includes four or more primary coils. In this example, the primary winding group 31 has four primary coils 41, 42, 43, and 44. Note that the number of primary coils and legs may be four or more.

[0177] Of the four legs, two legs 21 and 24 are wound with primary coils 41 and 44, respectively. The other two legs 22 and 23 are wound with only other primary coils 42 and 43, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound on the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0178] The primary winding group 31 is composed of two primary windings 31a and 31b. One primary winding 31a is composed of a primary coil 41 and another primary coil 42 connected to the primary coil 41. The other primary winding 31b is composed of a primary coil 44 and another primary coil 43 connected to the primary coil 44. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0179] The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other. That is, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2.

[0180] 16A, the number of turns of the primary coil 41 in the leg 21 is greater than the number of turns of the other primary coils 42 in the leg 22. Also, the number of turns of the primary coil 44 in the leg 24 is greater than the number of turns of the other primary coils 43 in the leg 23. This makes it possible to increase the magnetic flux generated by the primary winding group 31 that interlinks with the secondary winding group 32 in each of the legs 21 and 24, thereby reducing leakage inductance compared to the embodiment.

[0181] FIG. 16B is a schematic diagram illustrating another example of a transformer 1G according to the seventh modification of the embodiment.

[0182] 16B includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 and the secondary winding group 32 have the same configurations as those in the first modification.

[0183] The primary winding group 31 includes four or more primary coils. In this example, the primary winding group 31 has four primary coils 41, 42, 43, and 44. Note that the number of primary coils and legs may be four or more.

[0184] Of the four legs, two legs 22 and 23 are wound with primary coils 42 and 43, respectively. The other two legs 21 and 24 are wound with only the other primary coils 41 and 44, respectively. In other words, the primary coils 41, 42, 43, and 44 are wound on the legs 21, 22, 23, and 24 in one-to-one correspondence.

[0185] The primary winding group 31 is composed of two primary windings 31a and 31b. One primary winding 31a is composed of a primary coil 42 and another primary coil 41 connected to the primary coil 42. The other primary winding 31b is composed of a primary coil 43 and another primary coil 44 connected to the primary coil 43. Since the primary windings 31a and 31b are connected in series, these four primary coils 41 to 44 are also connected in series.

[0186] The winding directions of the two primary coils wound around the two adjacent legs in the second direction x2 are opposite to each other. That is, the primary coils 41 to 44 are wound in alternate opposite directions in the order in which they are arranged in the second direction x2.

[0187] 16B , the number of turns of the primary coil 42 in the leg 22 is greater than the number of turns of the other primary coils 41 in the leg 21. Furthermore, the number of turns of the primary coil 43 in the leg 23 is greater than the number of turns of the other primary coils 44 in the leg 24. This makes it possible to increase the magnetic flux that is generated by the primary winding group 31 and that interlinks with the secondary winding group 32 in each of the legs 22 and 23, and thus makes it possible to reduce leakage inductance compared to the first modification.

[0188] In the transformer 1G of the seventh modification, the number of turns of the primary coil in the leg on which the primary coil and the secondary coil are wound is greater than the number of turns of the primary coil in the other leg on which the primary coil is wound, which allows adjustment to reduce leakage inductance.

[0189] [Modification 8] A transformer 1H according to Modification 8 of the embodiment will be described. In Modification 8, an example will be described in which the leg portions of the core 10 are made of different materials.

[0190] FIG. 17 is a schematic diagram of a transformer 1H according to an eighth modification of the embodiment.

[0191] As shown in FIG. 17, a transformer 1H includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0192] The core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and four legs 21, 22, 23, and 24. The four legs 21 to 24 are arranged side by side along the second direction x2. Each of the legs 21 to 24 is provided between the first base 11 and the second base 12 and is in contact with the second base 12 in the first direction x1. In other words, each of the legs 21 to 24 is sandwiched between the first base 11 and the second base 12.

[0193] The configurations of the primary winding group 31 and the secondary winding group 32 of the eighth modification are the same as those of the seventh modification shown in Fig. 16A. In this modification as well, the number of turns of the other primary coils 42, 43 on the legs 22, 23 on which the other primary coils are wound is smaller than the number of turns of the primary coils 41, 44 on the legs 21, 24 on which the primary and secondary coils are wound.

[0194] In the eighth modification, the magnetic permeability of the legs 21 and 24 located on the outermost sides in the second direction x2 among the four legs 21 to 24 is lower than the magnetic permeability of the legs 22 and 23 located on the inner sides.

[0195] For example, the legs 22 and 23 are formed from an Mn-Zn ferrite material having an initial magnetic permeability of approximately 2000, and the legs 21 and 24, the first base 11 and the second base 12 are formed from an Ni-Zn ferrite material having an initial magnetic permeability of approximately 300.

[0196] However, without being limited to the above, the legs 22 and 23 may be formed from a Ni-Zn ferrite material having an initial magnetic permeability of approximately 300, and the legs 21 and 24, the first base 11, and the second base 12 may be formed from a metal composite material having an initial magnetic permeability of approximately 30. Alternatively, the legs 22 and 23 may be formed from a Mn-Zn ferrite material, and the legs 21 and 24, the first base 11, and the second base 12 may be formed from a metal composite material.

[0197] In the above description, the magnetic permeability of the outermost legs 21, 24 is lower than that of the innermost legs 22, 23, but this is not limited to this, and the magnetic permeability of the outermost legs 21, 24 may be higher than that of the innermost legs 22, 23.

[0198] In the transformer 1H of the eighth modification, the outermost leg in the second direction x2 and the leg positioned inside the outermost leg are made of materials with different magnetic permeabilities. By making the magnetic permeabilities of the legs different, the magnetic resistance of each leg can be changed. This allows a desired leakage inductance to be obtained.

[0199] [Modification 9] A transformer 1i according to Modification 9 of the embodiment will be described. In Modification 9, an example will be described in which the length of the legs of the core 10 is changed to change the distance between the second base portion 12 and the winding.

[0200] FIG. 18 is a schematic diagram of a transformer 1i according to a ninth modification of the embodiment.

[0201] 18, the transformer 1i includes a core 10, a primary winding group 31, and a secondary winding group 32. The basic configurations of the core 10, the primary winding group 31, and the secondary winding group 32 are the same as those in the embodiment.

[0202] Referring to the same figure, an example will be described in which the distance between the gap g and the winding is changed by changing the length of the legs 22, 23 while maintaining the number of turns of the other primary coils 42, 43. For example, shortening the length of the legs 22, 23 shortens the distance between the gap g and the ends (bottom ends in the figure) of the other primary coils 42, 43. Conversely, increasing the length of the legs 22, 23 increases the distance between the gap g and the ends of the other primary coils 42, 43.

[0203] 19 is a diagram showing the magnetic flux generated around the winding when a current is passed through it. In this figure, the magnitude of the magnetic flux density is represented by shades of black and white, with the whiter the color, the higher the magnetic flux density.

[0204] Figure 19(a) shows an example in which the leg length is set to 5 mm, making the leg length shorter. When the leg length is shorter, the distance between the gap g and the winding is closer, making the winding more susceptible to the influence of magnetic fields. On the other hand, Figure 19(b) shows an example in which the leg length is set to 8 mm, making the leg length longer than in (a). When the leg length is longer, the distance between the gap g and the winding is greater, making the winding less susceptible to the influence of magnetic fields.

[0205] FIG. 20 is a diagram showing the relationship between the length of the legs of the core 10 and copper loss.

[0206] The horizontal axis of the figure represents the length of the legs, and the vertical axis represents the copper loss of the winding. The copper loss of the winding shown here is mainly due to eddy current loss in the winding caused by leakage flux from the core 10. As shown in the figure, increasing the length of the legs reduces copper loss. Therefore, to reduce copper loss, it is desirable to increase the length of the legs and move the winding away from the gap g. Therefore, the winding is arranged in a region that overlaps with the legs in the first direction x1 but does not overlap with the gap g. In other words, the winding is arranged in a position that does not overlap with the gap g when viewed from a third direction x3 that is perpendicular to both the first direction x1 and the second direction x2.

[0207] In the transformer 1i of the ninth modification, a gap g is provided between the legs 21 to 24 and the first base 11 or between the legs 21 to 24 and the second base 12. The primary coils 41 to 44 and the secondary coils 51 to 54 are positioned so as not to overlap with the gap g when viewed from the third direction x3. This reduces eddy current loss in the primary coils 41 to 44 and the secondary coils 51 to 54 caused by leakage flux from the core 10.

[0208] [Modification 10] A transformer 1J according to Modification 10 of the embodiment will be described. Modification 10 describes an example in which adjacent coils partially overlap each other when viewed from the first direction x1.

[0209] Fig. 21A is a schematic diagram illustrating an example of a transformer 1J according to a tenth modification of the embodiment. Fig. 21B is a schematic diagram illustrating another example of the transformer 1J according to the tenth modification of the embodiment. Fig. 21C is a schematic diagram illustrating another example of the transformer 1J according to the tenth modification of the embodiment.

[0210] A transformer 1J shown in FIGS. 21A to 21C includes a core 10, a primary winding group 31, and a secondary winding group 32.

[0211] The core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11 and four legs 21, 22, 23, and 24. The second core 10b is composed of a second base 12. In other words, the core 10 has the first base 11, the second base 12, and the four legs 21 to 24. The four legs 21 to 24 are connected to the first base 11.

[0212] The primary winding group 31 and the secondary winding group 32 are formed inside and on the surface of the printed circuit board 70. The printed circuit board 70 is formed by stacking multiple sheet layers. A conductor pattern 80 is formed on each of the multiple sheet layers, and the conductor patterns 80 are connected between layers by via conductors (not shown). The primary winding group 31 and the secondary winding group 32 are formed by the conductor patterns 80, via conductors, and the like provided on the printed circuit board 70.

[0213] The printed circuit board 70 has four through holes. The legs 21 to 24 of the core 10 are inserted into the four through holes of the printed circuit board 70. By inserting the legs 21 to 24 into the four through holes, the conductor pattern 80 surrounds the legs, and each coil is essentially wound around each leg.

[0214] The primary winding group 31 has four primary coils 41, 42, 43, and 44. The secondary winding group 32 has four secondary coils 51, 52, 53, and 54. In this example, the primary coils 41 to 44 are formed by two layers of conductor patterns 80 including spiral planar coils. The secondary coils 51 to 54 are coils with a wiring width wider than that of the embodiment, and are formed by one layer of conductor patterns 80.

[0215] 21A, the distance between two adjacent legs of the four legs 21 to 24 is narrower than in the embodiment. When this transformer 1J is viewed from the first direction x1, the primary coil 41 and secondary coils 51 and 52 overlap the other primary coil 42 between the two legs 21 and 22. The other primary coil 42 overlaps the other primary coil 43 between the two legs 22 and 23. The other primary coil 43 overlaps the primary coil 44 and secondary coils 53 and 54 between the two legs 23 and 24.

[0216] 21B, the distance between two legs 21 and 22 and the distance between two legs 23 and 24 are narrower than the distance between two legs 22 and 23. When this transformer 1J is viewed from the first direction x1, the primary coil 41 and secondary coils 51 and 53 overlap with the other primary coil 42 between the two legs 21 and 22. Furthermore, the other primary coil 43 overlaps with the primary coil 44 and secondary coils 53 and 54 between the two legs 23 and 24.

[0217] 21C , the distance between two legs 22 and 23 is narrower than the distance between two legs 21 and 22 and the distance between two legs 23 and 24. When this transformer 1J is viewed from the first direction x1, the other primary coil 42 and the other primary coil 43 overlap between the two legs 22 and 23.

[0218] In transformer 1J of modification 10, two primary coils wound corresponding to two adjacent legs in the second direction x2 overlap between the two legs when viewed from the first direction x1. By overlapping a portion of adjacent coils in this manner, the area of ​​transformer 1J (the area in the x2-x3 plane) can be reduced, enabling miniaturization.

[0219] It is not necessary that the coils overlap at multiple locations, as long as adjacent coils overlap at least one location.

[0220] [Modification 11] A transformer 1K according to Modification 11 of the embodiment will be described. In Modification 11, an example will be described in which secondary coils are provided on both outer sides of the primary coil and bus bars are connected to the secondary coils.

[0221] FIG. 22 is a schematic diagram of a transformer 1K according to an eleventh modification of the embodiment.

[0222] 22 includes a core 10, a primary winding group 31, and a secondary winding group 32. The configurations of the core 10, the primary winding group 31, and the secondary winding group 32 are similar to those of the modified example 10 shown in FIG. 21A.

[0223] The secondary coils 51 and 52 of the transformer 1K are arranged on both outer sides of the primary coil 41 in the first direction x1. The secondary coils 53 and 54 are arranged on both outer sides of the primary coil 44 in the first direction x1.

[0224] The transformer 1K also has four bus bars 60 connected to the secondary coils 51 to 54. The bus bars 60 are formed of copper plates and have a U-shape when viewed from the first direction x1. The thickness of the bus bars 60 is, for example, 5 to 15 times the thickness of each of the secondary coils 51 to 54. Two of the four bus bars 60 are arranged on both outer sides of the secondary coils 51 and 53 in the first direction x1. The other two bus bars 60 are arranged on both outer sides of the secondary coils 53 and 54 in the first direction x1.

[0225] In the transformer 1K of the modification 11, the two secondary coils are arranged on both sides of the primary coil in the first direction x1. Therefore, the secondary coil, through which a large current flows and which is likely to generate heat, is arranged on the outside, thereby improving the heat dissipation performance of the transformer 1K.

[0226] Furthermore, in the transformer 1K of the eleventh modification, a bus bar 60 is connected to each of the secondary coils 51 to 54. This makes it possible to reduce the wiring resistance of the wiring connected to the secondary coils 51 to 54.

[0227] [Modification 12] A transformer 1L according to Modification 12 of the embodiment will be described. In Modification 12, an example in which a shielding layer 85 is provided on a printed circuit board 70 will be described.

[0228] Fig. 23A is a schematic diagram illustrating an example of a transformer 1L according to a twelfth modification of the embodiment. Fig. 23B is a schematic diagram illustrating another example of a transformer 1L according to a twelfth modification of the embodiment.

[0229] 23A and 23B includes a core 10, a primary winding group 31, and a secondary winding group 32. The core 10 and the secondary winding group 32 are similar to those of the modified example 10 shown in FIG. 21A.

[0230] The primary winding group 31 and the secondary winding group 32 are formed by a conductor pattern 80 and the like provided on a printed circuit board 70 .

[0231] In the twelfth modification, a plurality of shield layers 85 are provided within the printed circuit board 70. Each shield layer 85 is formed by a single layer of conductor pattern. The shield layer 85 is a floating electrode for the purpose of electrostatic shielding. The shield layer 85 may be connected to earth.

[0232] The transformer 1L shown in Figure 23A has four shield layers 85. The first shield layer 85 is provided between the secondary coil 51 and the primary coil 41. The second shield layer 85 is provided between the secondary coil 52 and the primary coil 41, and is arranged so as to overlap between the secondary coil 52 and another primary coil 42. The third shield layer 85 is provided between the secondary coil 53 and the primary coil 44, and is arranged so as to overlap between the secondary coil 53 and another primary coil 43. The fourth shield layer 85 is provided between the secondary coil 54 and the primary coil 44.

[0233] The transformer 1L shown in FIG. 23B has three shield layers 85. The shield layers 85 are provided parallel to the first base 11 over the entire area except for the legs 21 to 24. The first shield layer 85 is provided between the secondary coils 51 and 53 and the primary coil 41 and other primary coils 43. The second shield layer 85 is provided between the primary coil 41 and other primary coils 43 and the other primary coils 42 and primary coils 44. The third shield layer 85 is provided between the other primary coils 42 and 44 and the secondary coils 52 and 54.

[0234] The transformer 1L of the twelfth modification has a plurality of conductor patterns 80 and a plurality of shield layers 85. The plurality of shield layers 85 are provided between the plurality of conductor patterns 80. By providing the shield layers 85 between the conductor patterns 80 in this manner, it is possible to suppress leakage of electromagnetic field noise to the outside, for example, when the transformer 1L is operated at a high frequency.

[0235] The shield layer 85 does not need to be provided in multiple locations, but only needs to be provided in at least one location between the two conductor patterns 80 .

[0236] [Modification 13] A transformer 1M according to Modification 13 of the embodiment will be described. Modification 13 describes a specific example in which the primary coils 41 to 44 are formed by spiral planar coils.

[0237] Fig. 24 is a schematic diagram of a transformer 1M according to a thirteenth modification of the embodiment. Fig. 25 is a diagram showing a conductor pattern 80 and via conductors 82a to 82f of the transformer 1M.

[0238] The cross section of the transformer 1M shown in FIG. 24 is similar to that of the tenth modification shown in FIG. 21A.

[0239] Each of the primary coils 41 to 44 shown in Figure 25 is formed by a spiral planar coil. The arrows on the conductor pattern 80 shown in the figure indicate the direction of current flowing through the primary coils 41 to 44, and indicate the direction of current in the positive half cycle of the AC voltage waveform. When the winding direction of the primary coils 41 to 44 is viewed in the direction of current flow, the winding directions of the primary coils 41 to 44 are alternately reversed in the order in which they are arranged in the second direction x2. Each of the secondary coils 51 to 54 is formed by a U-shaped planar coil.

[0240] The coil of the transformer 1M shown in Fig. 24 is formed by a plurality of sheet layers. In this example, the coil is formed by stacking six sheet layers 71, 72, 73, 74, 75, and 76 in order in the direction from the first base 11 to the second base 12.

[0241] In the figure, the first sheet layer 71, which is the top layer, is formed with a 3 / 4-turn secondary coil 51 and a 3 / 4-turn secondary coil 53. The second sheet layer 72 is formed with a 3.5-turn primary coil 41 and a 3-turn primary coil 43. The third sheet layer 73 is formed with a 2.5-turn primary coil 41 and a 3-turn primary coil 43 connected in series. The primary coils 41 and 43 are connected because, for example, connecting adjacent primary coils 41 and 42 on the same sheet layer would prevent the two coils from being stacked vertically between legs 21 and 22, thereby increasing the length of the transformer in the second direction x2. The fourth sheet layer 74 is formed with a 3-turn primary coil 42 and a 2.5-turn primary coil 44. The fifth sheet layer 75 is formed with a 3-turn primary coil 42 and a 3.5-turn primary coil 44. The sixth sheet layer 76, which is the bottom sheet layer, has a 3 / 4 turn secondary coil 52 and a 3 / 4 turn secondary coil 54 formed thereon.

[0242] The connection relationship of the primary coils 41 to 44 will be described with reference to FIG. 25 . For example, the outer peripheral end of the primary coil 42 on the sheet layer 74 serves as a signal input / output point p1 for the primary coils 41 to 44. The inner peripheral end of the primary coil 42 on the sheet layer 74 is connected to the inner peripheral end of the primary coil 42 on the sheet layer 75 via a via conductor 82a. The outer peripheral end of the primary coil 42 on the sheet layer 75 is connected to the outer peripheral end of the primary coil 41 on the sheet layer 72 via a via conductor 82b. The inner peripheral end of the primary coil 41 on the sheet layer 72 is connected to the inner peripheral end of the primary coil 41 on the sheet layer 73 via a via conductor 82c. The primary coils 41 and 43 on the sheet layer 73 are connected in series, and the inner peripheral end of the primary coil 43 is connected to the inner peripheral end of the primary coil 43 on the sheet layer 72 via a via conductor 82d. The outer peripheral end of the primary coil 43 on the sheet layer 72 is connected to the outer peripheral end of the primary coil 44 on the sheet layer 75 through a via conductor 82e. The inner peripheral end of the primary coil 44 on the sheet layer 75 is connected to the inner peripheral end of the primary coil 44 on the sheet layer 74 through a via conductor 82f. The outer peripheral end of the primary coil 44 on the sheet layer 74 serves as a signal input / output point p2.

[0243] The transformer 1M of the thirteenth modification has via conductors 82a to 82f that connect multiple conductor patterns 80 for forming the primary coils 41 to 44. The via conductor 82c that connects two conductor patterns 80 in the primary coil 41 is disposed between both ends E, E of the conductor pattern 80 for forming the secondary coils 51 and 52 when viewed from the first direction x1. The via conductor 82f that connects two conductor patterns 80 in the primary coil 44 is disposed between both ends E, E of the conductor pattern 80 for forming the secondary coils 53 and 54 when viewed from the first direction x1. The "between both ends E, E" refers to the region between one end E and the other end E of the conductor pattern 80 that forms the secondary coil when the two ends E are connected by the shortest distance. The other via conductors 82a, 82b, 82d, and 82e are disposed in positions that do not overlap the secondary coils 51 to 54 when viewed from the first direction x1.

[0244] This makes it possible to maintain an insulation distance between the primary winding group 31 and the secondary winding group 32. This makes it possible to stabilize the operation of the LLC converter 5 using the transformer 1M.

[0245] [Modification 14] A transformer 1N according to Modification 14 of the embodiment will be described. Modification 14 will also describe a specific example in which the primary coils 41 to 44 are formed by spiral planar coils.

[0246] Fig. 26 is a schematic diagram of a transformer 1N according to a modification 14 of the embodiment. Fig. 27 is a diagram showing a conductor pattern 80 and via conductors 82a to 82f of the transformer 1N.

[0247] The cross section of a transformer 1N shown in FIG. 26 is an example in which the primary coil in the first modification shown in FIG. 21B has two turns.

[0248] Each of the primary coils 41 to 44 shown in Figure 27 is formed by a spiral planar coil. The arrows on the conductor pattern 80 shown in the figure indicate the direction of current flowing through the primary coils 41 to 44, and indicate the direction of current in the positive half cycle of the AC voltage waveform. When the winding direction of the primary coils 41 to 44 is viewed in the direction of current flow, the winding directions of the primary coils 41 to 44 are alternately reversed in the order in which they are arranged in the second direction x2. Each of the secondary coils 51 to 54 is formed by a U-shaped planar coil.

[0249] The coil of the transformer 1N shown in Fig. 26 is formed by a plurality of sheet layers. In this example, the coil is formed by stacking six sheet layers 71, 72, 73, 74, 75, and 76 in order in the direction from the first base 11 to the second base 12.

[0250] In the figure, the first sheet layer 71, which is the top layer, has a 3 / 4-turn secondary coil 51 and a 3 / 4-turn secondary coil 53 formed thereon. The second sheet layer 72 has a 2.5-turn primary coil 42 and a 2.5-turn primary coil 43 formed thereon. The third sheet layer 73 has a 3.5-turn primary coil 42 and a 3.5-turn primary coil 43 formed thereon. The fourth sheet layer 74 has a 3.5-turn primary coil 41 and a 3.5-turn primary coil 44 formed thereon. The fifth sheet layer 75 has a 2.5-turn primary coil 41 and a 2.5-turn primary coil 44 formed thereon. The sixth sheet layer 76, which is the bottom sheet layer, has a 3 / 4-turn secondary coil 52 and a 3 / 4-turn secondary coil 54 formed thereon.

[0251] The connection relationship of the primary coils 41 to 44 will be described with reference to FIG. 27 . For example, the outer peripheral end of the primary coil 42 of the sheet layer 72 serves as a signal input / output point p1 for the primary coils 41 to 44. The inner peripheral end of the primary coil 42 of the sheet layer 72 is connected to the inner peripheral end of the primary coil 42 of the sheet layer 73 via a via conductor 82a. The outer peripheral end of the primary coil 42 of the sheet layer 73 is connected to the outer peripheral end of the primary coil 41 of the sheet layer 74 via a via conductor 82b. The inner peripheral end of the primary coil 41 of the sheet layer 74 is connected to the inner peripheral end of the primary coil 41 of the sheet layer 75 via a via conductor 82c. The primary coils 41 and 44 of the sheet layer 75 are connected in series, and the inner peripheral end of the primary coil 44 is connected to the inner peripheral end of the primary coil 44 of the sheet layer 74 via a via conductor 82d. The outer peripheral end of the primary coil 44 on the sheet layer 74 is connected to the outer peripheral end of the primary coil 43 on the sheet layer 73 through a via conductor 82e. The inner peripheral end of the primary coil 43 on the sheet layer 73 is connected to the inner peripheral end of the primary coil 43 on the sheet layer 72 through a via conductor 82f. The outer peripheral end of the primary coil 43 on the sheet layer 72 serves as a signal input / output point p2.

[0252] The transformer 1N of Variation 14 has via conductors 82a-82f that connect multiple conductor patterns 80 that form the primary coils 41-44. The via conductor 82a that connects two conductor patterns 80 in the primary coil 42 is disposed between both ends E, E of the conductor pattern 80 that form the secondary coils 51, 52 when viewed from the first direction x1. The via conductor 82f that connects two conductor patterns 80 in the primary coil 43 is disposed between both ends E, E of the conductor pattern 80 that form the secondary coils 53, 54 when viewed from the first direction x1. The "between both ends E, E" refers to the region between one end E and the other end E of the conductor pattern 80 that forms the secondary coil when the two ends E are connected by the shortest distance. The other via conductors 82b, 82c, 82d, and 82e are disposed in positions that do not overlap the secondary coils 51-54 when viewed from the first direction x1.

[0253] This makes it possible to maintain an insulation distance between the primary winding group 31 and the secondary winding group 32. This makes it possible to stabilize the operation of the LLC converter 5 using the transformer 1N.

[0254] Another Example of the Embodiment An LLC converter 5A according to another example of the embodiment will be described.

[0255] FIG. 28 is a circuit diagram showing another example of the LLC converter 5A according to the embodiment.

[0256] The LLC converter 5A includes a primary circuit 6, a transformer 1, and a secondary circuit 7. The LLC converter 5A also includes a resonant capacitor C11 and an external inductor Lex. The external inductor Lex is provided on a path connecting the primary circuit 6 and the transformer 1. The external inductor Lex is connected in series with the transformer 1 and to the primary circuit 6 via the resonant capacitor C11.

[0257] As described above, the LLC converter 5A includes the above-described transformer 1, the external inductor Lex connected in series to the transformer 1, and the resonant capacitor C11 connected in series to the external inductor Lex.

[0258] This configuration enables soft switching operation of LLC converter 5A, allowing LLC converter 5A to operate with high efficiency and low noise. Note that the transformer used in LLC converter 5A is not limited to transformer 1, and the same applies to transformers 1A to 1N.

[0259] (Summary) Transformers and the like according to the embodiments are exemplified below.

[0260] The transformer of Example 1 includes a core 10 having a first base 11, a second base 12 opposed to the first base 11, and four or more legs (e.g., 21 to 24) between the first base 11 and the second base 12, and a primary winding group 31 and a secondary winding group 32 wound around the core 10. The four or more legs are spaced apart in a second direction x2 that intersects with a first direction x1 in which the first base 11 and the second base 12 face each other. The primary winding group 31 includes four or more primary coils (e.g., 41 to 44), and the secondary winding group 32 includes two or more secondary coils (e.g., 51 to 54). The four or more primary coils are wound corresponding to the four or more legs and connected in series. The winding directions of two primary coils wound corresponding to two adjacent legs in the second direction x2 are opposite to each other. Two or more of the four or more legs are wound with a primary coil and a secondary coil, respectively, and two or more other legs different from the two or more legs are wound with another primary coil different from the primary coil, respectively.

[0261] In the above example, of the two or more legs 21, 24, the leg 21 is wound with the primary coil 41 and the secondary coils 51, 52, and the leg 24 is wound with the primary coil 44 and the secondary coils 53, 54. Of the other two or more legs 22, 23, the leg 22 is wound with only the other primary coil 42, and the leg 23 is wound with only the other primary coil 43.

[0262] In another example, of the two or more legs 22, 23, the primary coil 42 and secondary coils 51, 52 are wound around leg 22, and the primary coil 43 and secondary coils 53, 54 are wound around leg 23. Of the other two or more legs 21, 24, only the other primary coil 41 is wound around leg 21, and only the other primary coil 44 is wound around leg 24.

[0263] In this way, by winding a primary coil and a secondary coil around each of two or more legs and winding other primary coils around each of the other two or more legs, the magnetic flux generated by the primary winding group 31 can be linked to the secondary coils wound around two or more legs without linking to the secondary coils in the other two or more legs. This makes it possible to generate magnetic fluxes of different magnitudes in multiple regions, thereby increasing the leakage inductance of the transformer.

[0264] The transformer of Example 2 is the transformer described in Example 1, wherein two or more secondary coils wound corresponding to two or more legs may be connected in parallel with each other.

[0265] This allows the magnetic flux generated by the primary winding group 31 to be linked to two or more secondary coils connected in parallel. This allows magnetic fluxes of different magnitudes to be generated in multiple regions, thereby increasing the leakage inductance of the transformer. Furthermore, connecting the secondary coils in parallel can suppress conduction loss in the secondary winding group 32.

[0266] The transformer of Example 3 is the transformer described in Example 1 or 2, in which magnetic flux is generated in each of the four or more legs when a current flows through the primary winding group 31. The primary winding group 31 may be wound around the four or more legs such that the magnetic fluxes generated in two legs adjacent to each other in the second direction x2 are reinforced by each other.

[0267] In this way, by winding the primary winding group 31 on the legs so that the magnetic fluxes generated in two adjacent legs are generated in directions that reinforce each other, it is possible to increase the magnetic flux generated by the primary winding group 31. This makes it possible to increase the leakage inductance of the transformer.

[0268] A transformer of Example 4 is the transformer according to any one of Examples 1 to 3, in which the primary winding group 31 is configured with two primary windings 31a, 31b. The secondary winding group 32 is configured with two secondary windings 32a, 32b corresponding to the two primary windings 31a, 31b in a one-to-one relationship. Each of the two primary windings 31a, 31b is configured with a primary coil and another primary coil connected in series. Each of the two secondary windings 32a, 32b may be configured with two secondary coils connected in parallel.

[0269] This allows the magnetic flux generated by the primary winding group 31 to be linked to the two secondary coils connected in parallel. This allows magnetic fluxes of different magnitudes to be generated in multiple regions, increasing the leakage inductance of the transformer. Furthermore, connecting the secondary coils in parallel reduces conduction loss in the secondary winding group 32.

[0270] The transformer of Example 5 is the transformer described in any one of Examples 1 to 4, wherein the core 10 may have a first core 10a including a first base 11 and four or more legs, and a second core 10b including a second base 12.

[0271] By including four or more legs in the core 10, the magnetic flux generated by the primary winding group 31 can be linked to the secondary coil wound around two or more legs with a simple structure. This allows magnetic fluxes of different magnitudes to be generated in multiple regions, thereby increasing the leakage inductance of the transformer.

[0272] A transformer of Example 6 is the transformer according to any one of Examples 1 to 5, in which the four or more legs are arranged side by side along the second direction x2. A distance Ia in the first direction x1 between an outermost leg in the second direction x2 among the four or more legs and the first base 11 or the second base 12, and a distance Ib in the first direction x1 between an innermost leg in the second direction x2 and the first base 11 or the second base 12 may be different from each other.

[0273] By making the interval Ia and the interval Ib different in this way, a desired leakage inductance can be obtained.

[0274] The transformer of Example 7 is the transformer according to any one of Examples 1 to 6, in which the four or more legs are arranged side by side along the second direction x2. At least one of the cross-sectional areas of the outermost legs in the second direction x2, the cross-sectional area of ​​the legs positioned more inward than the outermost legs, the cross-sectional area of ​​the first base 11 in a plane perpendicular to the second direction x2, and the cross-sectional area of ​​the second base 12 in a plane perpendicular to the second direction x2 may be different.

[0275] By varying the cross-sectional area of ​​the core 10 in this manner, a desired leakage inductance can be obtained.

[0276] The transformer of Example 8 is the transformer according to any one of Examples 1 to 7, wherein the number of turns of the primary coil in the leg on which the primary coil and the secondary coil are wound may be different from the number of turns of the primary coil in the leg on which the other primary coil is wound.

[0277] By varying the number of turns of the primary coil in this way, a desired leakage inductance can be obtained.

[0278] The transformer of Example 9 is the transformer according to any one of Examples 1 to 8, wherein the number of turns of the primary coil in the leg on which the other primary coil is wound may be less than the number of turns of the primary coil in the leg on which the primary coil and the secondary coil are wound.

[0279] This allows the leakage inductance to be adjusted by the number of turns of the primary coil.

[0280] A transformer of Example 10 is the transformer according to any one of Examples 1 to 9, wherein the four or more legs are arranged side by side along the second direction x2. Of the four or more legs, the outermost leg and the leg located more inward than the outermost leg in the second direction x2 may be made of materials having different magnetic permeabilities.

[0281] By varying the magnetic permeability of the materials that make up the legs in this way, a desired leakage inductance can be obtained.

[0282] The transformer of Example 11 is the transformer according to any one of Examples 1 to 10, wherein a gap g is provided between the leg and the first base 11 or between the leg and the second base 12. Each of the four or more primary coils and the two or more secondary coils may be arranged at a position that does not overlap with the gap g when viewed from a third direction x3 that is orthogonal to both the first direction x1 and the second direction x2.

[0283] This reduces the eddy current loss in the primary coil or secondary coil, thereby achieving a large leakage inductance.

[0284] The transformer of Example 12 is a transformer according to any one of Examples 1 to 11, in which the two primary coils wound corresponding to two adjacent legs in the second direction x2 may overlap between the two legs when viewed from the first direction x1.

[0285] This allows the area of ​​the transformer to be reduced, making it possible to make it smaller.

[0286] The transformer of Example 13 is a transformer described in any one of Examples 1 to 12, and the primary winding group 31 and the secondary winding group 32 may be formed by a conductor pattern 80 provided on a printed circuit board 70.

[0287] This allows the transformer to be formed with a simple structure.

[0288] The transformer of Example 14 is the transformer described in Example 13, in which the printed circuit board 70 is formed by stacking a plurality of sheet layers. The plurality of sheet layers has a plurality of conductor patterns 80 and one or more shield layers 85. The one or more shield layers 85 may be provided between the plurality of conductor patterns 80.

[0289] This makes it possible to suppress electromagnetic field noise from leaking out when the transformer is operated at high frequency, for example.

[0290] A transformer of Example 15 is the transformer of Example 13 or 14, and includes via conductors 82a to 82f that connect the plurality of conductor patterns 80 that form the primary coil. The via conductor (e.g., 82f) that forms the primary coil of the leg around which the primary coil and the secondary coil are wound may be disposed between both end portions E, E of the conductor patterns that form the secondary coil when viewed from the first direction x1.

[0291] This makes it possible to maintain an insulation distance between the primary winding group 31 and the secondary winding group 32. This makes it possible to stabilize the operation of the LLC converter 5 that uses a transformer.

[0292] A transformer of Example 16 is the transformer of Examples 1 to 15, wherein each of the two or more legs is wound with two secondary coils. The two secondary coils may be disposed on both outer sides of the primary coil in the first direction x1.

[0293] This allows the secondary coil, through which a large current flows, to be positioned on the outside where heat can be easily dissipated, thereby improving the heat dissipation performance of the transformer.

[0294] The transformer of Example 17 may be the transformer described in Example 16, in which a bus bar 60 is connected to the secondary coil.

[0295] By providing the bus bar 60 to the transformer in this manner, the wiring resistance of the wiring connected to the secondary coil can be reduced.

[0296] A DC-DC converter of Example 18 includes the transformer according to any one of Examples 1 to 17, and a resonant capacitor C11 connected in series with the transformer.

[0297] By using a transformer with a large leakage inductance as described above, the gain of the DC-DC converter can be made to have a wide range.

[0298] A DC-DC converter of Example 19 includes the transformer according to any one of Examples 1 to 17, an external inductor Lex connected in series with the transformer, and a resonant capacitor C11 connected in series with the external inductor Lex.

[0299] This configuration enables soft switching of the DC-DC converter, which allows the DC-DC converter to operate with high efficiency and low noise.

[0300] (Other Embodiments, etc.) While the transformer according to the present disclosure has been described above based on the embodiments, the present disclosure is not limited to the above-described embodiments.

[0301] In the above embodiment, the first base 11 and the second base 12 are rectangular and the legs are arranged in sequence along the linear second direction x2. However, this is not limiting. For example, the second direction x2 may be a curved direction. In this case, the first base 11 and the second base 12 are curved along the second direction x2, and the legs are arranged in sequence along the curved second direction x2.

[0302] In addition, this disclosure also includes forms obtained by applying various modifications to the above-mentioned embodiments that a person skilled in the art would conceive, and forms realized by arbitrarily combining the components and functions of each embodiment within the scope of the present disclosure.

[0303] The transformer according to the present disclosure is useful as a transformer used in a power source for an on-vehicle auxiliary device.

[0304] 1, 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1i, 1J, 1K, 1L, 1M, 1N Transformer 5 LLC converter (DC-DC converter) 6 Primary circuit 7 Secondary circuit 10 Core 10a First core 10b Second core 11 First base 12 Second base 21, 22, 23, 24, 25, 26, 27 Legs 31 Primary winding group 31a, 31b Primary winding 32 Secondary winding group 32a, 32b Secondary winding 41, 42, 43, 44, 45, 46, 47 Primary coil 51, 52, 53, 54, 55 Secondary coil 60 Bus bar 70 Printed circuit board 71, 72, 73, 74, 75, 76 Sheet layer 80 Conductive pattern 82a, 82b, 82c, 82d, 82e, 82f Via conductor 85 Shield layer C11 Resonant capacitor Cr Capacitance E End g Gap Ia, Ib Spacing Lex External inductor Lm, Lma, Lmb Excitation inductance Lr, Lra, Lrb Leakage inductance mc1, mc2, mc3, mc4, mc5, mc6 Magnetic circuit p1, p2 Input / output points p11, p12 Primary port p21, p22 Secondary port s11, s12, s13, s14, s21, s22, s23, s24 Switch element x1 First direction x2 Second direction x3 Third direction φa, φa1, φa2, φa3, φa4, φb, φb1, φb2, φb3 magnetic flux

Claims

1. A coil winding comprising: a core having a first base, a second base provided opposite the first base, and four or more legs provided between the first base and the second base; and a primary winding group and a secondary winding group wound around the core, wherein the four or more legs are arranged at intervals in a second direction intersecting a first direction in which the first base and the second base face each other; the primary winding group includes four or more primary coils; the secondary winding group includes two or more secondary coils; the four or more primary coils are wound corresponding to the four or more legs and connected in series; the winding directions of each of the two primary coils wound corresponding to two adjacent legs in the second direction are opposite to each other; and the primary coil and the secondary coil are wound around each of two or more legs of the four or more legs, A transformer, wherein another primary coil different from the primary coil is wound on each of two or more legs other than the two or more legs among the four or more legs.

2. The transformer according to claim 1, wherein the two or more secondary coils wound corresponding to the two or more legs are connected in parallel with each other.

3. A transformer as described in claim 1, wherein when a current is passed through the primary winding group, a magnetic flux is generated in each of the four or more legs, and the primary winding group is wound around the four or more legs such that the magnetic fluxes generated in two adjacent legs in the second direction are intensified directions.

4. A transformer as described in claim 1, wherein the primary winding group is constituted by two primary windings, the secondary winding group is constituted by two secondary windings corresponding one-to-one to the two primary windings, each of the two primary windings is constituted by the primary coil and the other primary coil connected in series, and each of the two secondary windings is constituted by the two secondary coils connected in parallel.

5. A transformer according to any one of claims 1 to 4, wherein the core comprises a first core including the first base portion and the four or more legs, and a second core including the second base portion.

6. A transformer as claimed in any one of claims 1 to 4, wherein the four or more legs are arranged side by side along the second direction, and a distance in the first direction between a leg located outermost in the second direction among the four or more legs and the first base or the second base, and a distance in the first direction between a leg located more inward than the outermost and the first base or the second base are different from each other.

7. A transformer as described in any one of claims 1 to 4, wherein the four or more legs are arranged side by side along the second direction, and at least one of the cross-sectional areas of the legs located outermost in the second direction among the four or more legs, the cross-sectional area of ​​the legs located more inside than the outermost, the cross-sectional area of ​​the first base in a plane perpendicular to the second direction, and the cross-sectional area of ​​the second base in a plane perpendicular to the second direction is different.

8. A transformer as claimed in any one of claims 1 to 4, wherein the number of turns of the primary coil in the leg on which the primary coil and the secondary coil are wound is different from the number of turns of the primary coil in the leg on which the other primary coil is wound.

9. A transformer as claimed in any one of claims 1 to 4, wherein the number of turns of the primary coil in the leg on which the other primary coil is wound is less than the number of turns of the primary coil in the leg on which the primary coil and the secondary coil are wound.

10. A transformer as described in any one of claims 1 to 4, wherein the four or more legs are arranged side by side along the second direction, and among the four or more legs, the leg located outermost in the second direction and the leg located more inside than the outermost are formed from materials having different magnetic permeabilities.

11. A transformer as described in any one of claims 1 to 4, wherein a gap is provided between the leg and the first base or between the leg and the second base, and each of the four or more primary coils and the two or more secondary coils is positioned so as not to overlap with the gap when viewed from a third direction perpendicular to both the first direction and the second direction.

12. A transformer as described in any one of claims 1 to 4, wherein the two primary coils wound corresponding to two adjacent legs in the second direction overlap between the two legs when viewed from the first direction.

13. A transformer according to any one of claims 1 to 4, wherein the primary winding group and the secondary winding group are formed by conductor patterns provided on a printed circuit board.

14. The transformer according to claim 13, wherein the printed circuit board is formed by laminating a plurality of sheet layers, the plurality of sheet layers having a plurality of the conductor patterns and one or more shielding layers, and the one or more shielding layers are provided between the plurality of the conductor patterns.

15. A transformer as described in claim 13, further comprising via conductors connecting a plurality of the conductor patterns for forming the primary coil, and the via conductors for forming the primary coil of the leg around which the primary coil and the secondary coil are wound are disposed between both ends of the conductor patterns for forming the secondary coil when viewed from the first direction.

16. A transformer as described in any one of claims 1 to 4, wherein two of the secondary coils are wound around each of the two or more legs, and the two secondary coils are arranged on either side of the primary coil in the first direction.

17. The transformer according to claim 16, wherein a bus bar is connected to the secondary coil.

18. A DC-DC converter comprising: a transformer according to any one of claims 1 to 4; and a resonant capacitor connected in series to the transformer.

19. A DC-DC converter comprising: a transformer according to any one of claims 1 to 4; an external inductor connected in series to the transformer; and a resonant capacitor connected in series to the external inductor.

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